Re: how do you determine the sugar content of an apple?

The most common sugar measurement used for fruit is soluble sugar, or the 
amount of sugar in juice of the apple.  This method only requires a small 
amount of juice and is very fast.

The tool used to measure soluble sugar is a refractometer.  The following 
comes from the catalog of the Cole-Parmer Instrument Company

	How a refractometer works:  When light enters a liquid
	it changes direction; this is called refraction.
	Refractometers measure the degree to which the light
	changes direction, called the angle of refraction.  A
	refractometer takes the refraction angles and correlates 
	them to the refractive index (nD) values that have been 
	established. Using these values, you can determine the 
	concentrations of solutions.  For example, solutions
	have different refractive indexes depending on their 
	concentration in water.  

	The prism in the refractometer has a greater refractive 
	index than the solution.  Measurements are read at the 
 	point where the prism and solution meet.  With a low 
	concentration in solution, the refractive index of the 
	prism is much greater than that of the sample, creating	
	a large refraction angle and a low reading.  The reverse
	would happen with a high concentrated solution.

Sugar is measured in Brix, which is a scale calibrated to the number of 
grams of cane sugar contained in 100 ml of water.  Therefore, the Brix % 
reading equals actual sugar concentration.  In an apple the Brix is 
typically 11 to 18 percent.

As you can see, the principle is fairly simple.  

I hope this helps.

Eric Biddinger
Graduate student
Department of Horticulture
Penn State University